boards: st: stm32h7s3l8 nucleo board supporting xspi instance

Add the XSPI 2 which is an octoSPI connection to a octo NOR flash
MX25UW25645GXDI00 (256 Mbits, 1.8 V, 200 MHz, DTR, rww)
on the nucleo_h7s3l8 board.
Use the STM32Cube programmer to flash with the corresponding
external loader for XiP mode.

Signed-off-by: Francois Ramu <francois.ramu@st.com>

Author: FRASTM

boards/st/nucleo_h7s3l8/nucleo_h7s3l8.dts

@@ -55,6 +55,14 @@
 		watchdog0 = &iwdg;
 		spi-flash0 = &mx25uw25645;
 	};
+
+	ext_memory: memory@70000000 {
+		compatible = "zephyr,memory-region";
+		reg = <0x70000000 DT_SIZE_M(64)>;
+		zephyr,memory-region = "EXTMEM";
+		/* The ATTR_MPU_EXTMEM attribut causing a MPU FAULT */
+		zephyr,memory-attr = <( DT_MEM_ARM(ATTR_MPU_IO) )>;
+	};
 };
 
 &clk_lsi {
@@ -186,6 +194,7 @@
 		ospi-max-frequency = <DT_FREQ_M(50)>;
 		spi-bus-width = <XSPI_OCTO_MODE>;
 		data-rate = <XSPI_DTR_TRANSFER>;
+		four-byte-opcodes;
 		status = "okay";
 
 		partitions {